Pressure operated switch



y 1947. H. 1.. BOWDITCH PRESSURE OPERATED SWITCH Filed April 21, 1944 INVENTOR. /70/ L Bowaffcb ATT NEYS Patented May 6, 1947 PRESSURE OPERATED SWITCH Eioel L. Bowditch, Sharon, Mass, assignor to The Foxboro Company, Fcxboro, Mass, a corporation of Massachusetts Application April 21, 1944, Serial No. 532,191

2 Claims.

This invention relates to indicating, recording and controlling apparatus and. more particularly to alarm systems therefor to indicate high or low pressure, temperature or flow conditions or explosive mixtures, etc.

Such apparatus contains a, measuring element which moves in accurate response to changes in the condition being indicated, recorded or controlled. But these measuring elements may have very little power to exert, in response to slight changes in the condition, to move even delicate, substantially frictionless, parts performing indieating or controlling functions. It is an object of the present invention to provide an alarm system for this apparatus without loading or otherwise binding or interfering with the normal action of the measuring element in its responsive movements. It is a further object to accomplish such object without any mechanical connection between the measuring element and the alarm system.

In the drawings, in which one satisfactory form of the invention is used by way of illustration:

Figure 1 is a perspective view taken at an angle from :below and with parts broken away, of a thermometer embodying the invention;

Figure 2 is a fragmentary elevational View with parts in section of a switch portion of the thermometer illustrated in Figure l, the switch being open; and

Figure 3 is a view similar to that of Figure 2 but showing the parts in the position which they assume when the switch is closed.

The invention accordingly consists in the fea tures of construction, combinations of elements, and arrangements of parts as will be exemplified by the structure to be hereinafter described and the scope of the application of which will be set forth in the accompanying claims.

In the embodiment here described, the invention is applied to an indicating thermometer. This thermometer is of a simple conventional construction wherein a pointer is actuated by the movement of the free end of a Bourdon tube in accordance with temperature changes. Such a system is advantageous in that it may be constructed to respond accurately to very small changes in temperature. It has the disadvantage of such systems, however, of providing only a relatively weak actuating force to operate the pointer for small temperature changes. Thus, whereas the free end of the Bourdon tube will respond very accurately, when unrestrained, to changes in condition at the thermometer bulb,

the tube is so lacking in power that if the pointer mechanism is not made substantially frictionless, then the accuracy of the system is lost. And if a reliable alarm system is to be included and actuated by the Bourdon tube the construction of the alarm must be such as not to impose drag or friction on the movement of the free end of the Bourdon tube-otherwise the alarm system will introduce inaccuracy into the indicating mechanism and into the alarm system.

In Figure 1, a Bourdon tube 20 is indicated attached to a bracket 22 by means of brazing 24. Bracket 22 is secured by screws 26 and 21 to a part of an instrument case not shown. The Bourdon tube is connected through capillary tubing '28 to a temperature responsive thermometer bulb not shown. The Bourdon tube, capillary tubing and thermal bulb are filled with fluid and sealed. Variation of the temperature of the fluid in the bulb varies the pressure in the system, and the Bourdon tube, which is the condition measuring element, moves in. accordance With the pressure variation.

The free end of the Bourdon tube 20 carries a clip 35) provided with a conventional seat-forming depression not shown) which cooperates with a ball-shaped portion (not shown) of lever 32. Lever 32 is threaded into a collar on a shaft 38 which also supports thereupon a sector gear The shaft 3% is mounted in bearin s and carried between plates to and M which are held in spaced-apart relationship by means of columns t? and i3, and screws ed and A hairspring 56 is fastened to column idas at es, and imparts a clockwise bias to shaft 33 so that balltipped lever is held in light engagement with the seat-forming depression in the surface of clip Sector gear 34 meshes with a pinion 36 on pointer shaft 562. Hence, as lever 32 responds to movements of the Bourdon tube, the movements are transferred by segment gear 35 and pinion 36 to the pointer shaft 50.

As shown in figure 1, the upper end of the shaft Eli carries extension shaft to which the pointer S is fastened. The extension shaft is supported by the shaft by means of a tapered press fit which in service causes the pointer to bear a fixed and unvarying relation. to the shaft and the pinion it. However, the extension. shaft 6% is removable for the purpose of performing repairs on the instrument.

The extension shaft 651) extends through a hole (hidden by setting wheel 58) in dial Ei a and the index pointer 59 serves to indicate with the index on the dial 6 the value of dition being measured,

Also carried by the extension tfib is a small magnet 52 which serves, when turned correctly (Figures 1 and 3) to close a conventional mercury switch 62 which is part of an alarm system. Operation of switch 62 actuates the alarm, The switch is supported between a clip and a bracket and includes an operating flapper 63 (see also Figures 2 and 3) carried by a hairspring 1- Referrin' to Figures 2 and 3 the operating carries a rightwardly extending contact wire lfi, movable into and out of a pool of mercury 73. A guide ha directs the to and fro path of the contact wire '56. ctric current is s; Jplled to the contact wire through a terminal wire it, a lead 3g iii and the operating flapper continues through the mercury pool and a return terminal w o a part of a operated -l rm system.

shows he constand net is securely l' u bu hing "iction tight setting wheel and which is of the gular osition er til. e at any positioning pointer 68. indicates the -e magnet.

observed that correct operation of c. mercury switch illustrated requires mercury switch always be positioned axis parallel to the pull of y in or er that the pool of mercury 1' main 1"531V6ll in the bottom of the en- Tie mounting of magnet .pon sleeve n that the axis about which the magnet the setti: t

the center of gravity of the magnet, and therefore. the efiect of the mass of he magnet on the movement of the shaft 55 reduced to a n ..um. This relationship has the additional advantage hat the pull of the magnet upon the is made negligible. Therefore, such construction is useful in indicators and controllers of the type above-mentioned because the addition of the magnet and switch and accompanying alarm system does not impose upon the instrument the sluggishness or inaccuracy which was inherent in most of the alarz- ,iving instruments heretofore available.

Other conventional condition sensitive measr""1g elements may be used to rotate the shaft by suitable connections. Thus floats, diaphragm bellows, solenoids, etc, might be used all within the scope of the invention.

In alternative applications of the present invention, the alarm may advantageously be combined with an industrial process controller, where the need for accurate response to process condition changes may be paramount. The addition of the alarm system does not interfere with the desired control function of the controller.

The present invention thus makes provision for an alarm system which will close a circuit ringing a bell, or lighting a light, or in some other manner attracting the attention of an operator when a critical condition is reached. A part of value of the invention resides in the fact that it may be incorporated in delicately responsive systems such as above mentioned without introducing error in the indicating, recording or controlling functions.

As many possible embodiments may be made of the above invention without departing from the scope thereof, and as changes will be necessitated in adapting the various embodiments to specific applications, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as i1- lustrative and not in a limiting sense.

I claim:

1. In apparatus for indicating, recording, or controlling a condition and employing a frictionless shaft rotatable through a range by a condition responsive element to positions corresponding to values of the condition, in combination, a switch including a magnetically operated switching member normally yieldingly urged by a light force to occupy one of two positions, and a magnet mounted on and movable by said frictionless shaft into and out of a switch operating position in which position the pull of the magnet on said switching member overcomes said force and moves the switching member to occupy the second of its two positions whereas when said magnet is not in said switch operating position the pull of the magnet does not operate to overcome said force and the switching member is held in the first of said two positions, whereby said switch is operated, when said condition responsive member moves the magnet into and out of said switch operating position, the center of gravity of said magnet falling substantially upon the axis about which said frictionless shaft rotates.

2.. In apparatus for indicating, recording, or controlling a condition and employing a frictionless shaft rotatable through a range by a condi tion responsive element to positions correspond-- ing to values of the condition, in combina ion. a. switch including a magnetically operated switching member normally yieldingly urged by a light force to occupy one of two positions. a magi t mounted on and movable by said f1 unless shaft into and out of a switch operating osition in which position the pull of the magp on said switching member overcomes said 101cc and moves the switching member to occupy the sec- 5 0nd of its two positions whereas when said mag- REFERENCES CITED net is in Said switch operating position the The following references are of record in the pull of the magnet does not operate to overcome me of this ate said force and the switching member is held. in A p the first of said two positions, whereby said 5 UNITED STATES PATENTS switch is operated as said condition responsive Nu b N t member moves the magnet into and out of said 2,109,953 Bates 1, 193 switch operating position, and a member for 2,226,287 Miller Dec, 24, 1940 manually shifting said magnet with respect to 2,333,263 McCabe Nov. 2, 1943 said frictionless shaft to adjust the value of the 10 75,313 Stern Mar, 10, 1868 condition that will cause the switch to operate. 2,203,512 Wesemann June 4, 1940 2,012,153 Bates Aug. 20, 1935 HOEL L. BOWDITCH. 2,251,129 Hammond July 29, 1941 

